Conventional methods for fabricating thermoplastic polymeric materials into shaped articles include extrusion, casting, injection molding and other hot-forming techniques. These techniques commonly involve three basic steps: (1) melting or softening the thermoplastic material; (2) shaping the molten or softened polymer with or without pressure in a mold cavity, in a press or through a die; and (3) cooling the shaped article in its final shape. However, this procedure becomes cumbersome when processing thick sections and is totally unsuitable when working with very viscous, ultra-high molecular weight polymers or those with very high melting points. On the other hand, interest in these two latter categories of thermoplastics is growing rapidly because of their unique thermal and mechanical properties.
Cold-forming, i.e., shaping a material below its melting point, is a processing technique that has been well developed in metallurgy but only recently applied in the field of polymers. Most of these recent polymer applications involve stamping and forging, machining, deep drawing, cold rolling, or cold extrusion. In all these processes, a starting material in the shape of a sheet or billet of relatively thick cross-section is required, which is itself usually prepared by hot extrusion. The combination of hot-forming followed by cold-forming and, in the case of machining, production of scrap which may or may not be reusable, adds to the cost of the overall shaping operation and represents a significant engineering and economic drawback. Nevertheless, there are considerable incentives to applying cold-forming techniques to shaping thermoplastics. For instance, parts are shaped entirely in the solid state, and since there is therefore no phase change which would otherwise cause shrinkage and distortion, adherence to strict dimensional tolerances is facilitated. Also, enhancement of certain engineering properties of the material is often realized.
Generally, for a thermoplastic polymer material to be formable in the solid phase, it must have ductility and strength. Materials of this type which have been cold formed include acrylonitrile-butadiene-styrene copolymers (ABS resins), cellulose acetate-butyrate, polycarbonates, polysulfones, polyvinylchloride (PVC) and polyolefins (e.g., high molecular weight, high density polyethylene). Most such forming operations take place 10.degree.-20.degree. C. below the melting point or glass transition temperature of the polymer.
Powder processing technology has been fully developed for metals, where it has in many instances shown itself to be more attractive than hot forging and melt processing, i.e., casting. In the polymer field, however, only a relatively few investigations, of a preliminary nature, have been made, as exemplified in the following publications:
D. M. Bigg, "High-Pressure Molding of Polymeric Powders," 33rd Annual Technical Conference, Society of Plastics Engineers, p. 472 (1975); PA0 M. A. Rudner, "Fluorocarbons" (Reinhold 1958); PA0 G. W. Halldin and I. L. Kamel, "Powder Processing of Ultrahigh Molecular Weight Polyethylene, I. Powder Characterization and Compaction," Polymer Engineering and Science, 17(1), 21 (1977); PA0 G. W. Halldin and I. L. Kamel, "Powder Processing of Ultrahigh Molecular Weight Polyethylene, II. Sintering," 35th Annual Technical Conference, Society of Plastics Engineers, 298 (1977); PA0 G. S. Jayaraman, J. F. Wallace, P. H. Geil and E. Baer, "Cold Compaction Molding and Sintering of Polystyrene," Polymer Engineering and Science, 16(8), 529 (1976); PA0 U.S. Pat. No. 2,067,025 (1937) to Schmidt for "Method of Transforming Polymerized Vinyl Chloride Into Thin Sheets and Product Obtainable Thereby"; PA0 U.S. Pat. No. 2,528,529 (1950) to Lyon for "Method Of and Apparatus For Forming Plastic"; PA0 U.S. Pat. No. 2,920,349 (1960) to White for "Polyethylene Films"; and PA0 U.S. Pat. No. 2,928,133 (1960) to Schairer for "Method Of Producing Sheet Material."
Specialty polymers such as ultra-high molecular weight polyethylene (UHMW-PE), poly(tetrafluoroethylene) and poly(benzimidazole) are receiving increasingly greater attention because of their unique mechanical and/or thermal properties. Unfortunately, these properties also limit the processability of such polymers by conventional hot- and cold-forming techniques. On the other hand, powder-forming techniques would seem to offer attractive alternatives to the problem of shaping such materials. As indicated previously, conventional powder processing has been used to a very limited extent for shaping thermoplastic polymers but has not been proven capable of widespread commercial applicability. A need therefore exists for improved powder processing techniques which can take full advantage of the properties of polymers in general and the unique properties of the aforesaid specialty materials in particular, to produce non-tearable, thin film and sheet at high overall rates of production.
Accordingly, it is an object of the present invention to provide new processes for producing shaped articles in the form of films directly from thermoplastic polymer powders.
Another object is to provide shaped thermoplastic articles in the form of films having improved properties and which have been formed directly from thermoplastic polymer powders.
Yet another object is to provide an apparatus for producing shaped articles in the form of films directly from thermoplastic polymer powders.
These and other objects of the invention, as well as a fuller understanding of the utility and advantages thereof, can be had by reference to the following disclosure and claims.